Perfect Quote

1
Perfect Quote

• You know you've achieved perfection in design,
• Not when you have nothing more to add,
• But when you have nothing more to take away.
• - Antoine de Saint Exupery.

2
Language Design

• One thing the language designer should not do is
  to include untried ideas of ones own. The task is
  one of consolidation, not innovation.
  - C. A.
  R. Hoare
• In language design, the central problem is to
  organize a variety of concepts in a way which
  exhibits uniformity and generality.
  - J. C. Reynolds
  

3

• Conflicting goals lead to difficult trade-offs.
• Safety checks burden the compiler and/or the
  run-time.
• Safety and machine independence may exclude
  efficient low-level operations.
• Type systems restrict programming style in
  exchange for strong guarantees.

4

• A language addition should whenever possible
  solve new problems beyond its original purpose
  --- not create new problems through its
  interaction with other mechanisms. A prudent
  language designer, having toyed with a possible
  new facility, and encountering incompatibilities
  with more important properties of the design,
  quickly retreats.
• -- Bertrand Meyer

5
Language Specification

• Programs
• (dynamic) Software Machinery
• (static) Naming Environments
• What entities can be named?
• How are the names organized into groups?
• How are the names made known where ever they are
  needed?
• Conventions are no substitute for enforced
  constraints.

6
Java Design Goals

• A general-purpose concurrent
• object-oriented language
• for the Internet.

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Top Ten Design Goals
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Simplicity

• Programmer Perspective
• Automatic garbage collection.
• Unconstrained array type.
• Implementer Perspective
• Only single class inheritance.
• Restricted overloading.
• E.g., Pascal Modula-2 Modula-3.

9
Portability

• Hardware Independence
• Java source Architecture Neutral Byte code
• Operating System Independence
• Java APIs for I/O, networking, GUI, concurrent
  programming, etc.

10
Portability

• Architecture-neutral.
• Size of integer, char, etc and meaning of
  floating point operations fixed by the language
  (not by an implementation).
• All behavioral aspects of a Java program
  defined by the Java language spec.
• Order of evaluation of operands fixed.
• Error and Exception handling.

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Object-Oriented Programming

• Programming with abstract data types
• Data Abstraction
• Separation of Spec. and Impl.
• Modularity
• Logically related entities are textually close.
• Encapsulation
• Controlling visibility of names.
• Code and Interface Reuse
• Inheritance Dynamic binding

12
Java

• Class-based language.
• Has object types (based on classes).
• Subclassing implies subtyping
• but there is subtyping independent of
  subclassing.
• Static typing

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Reliability and Robustness

• Strong Typing
• every variable and every expression has a type
  known at compile-time.
• type limits the values a variable can hold and
  the values an expression can produce at run-time.
• Exceptions
• signaling violation of semantic constraints.

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Java Compiler and Interpreter
source code
javac
byte code
java
native code
mips
pentium
sparc
alpha
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Interpretation and Security

• Java programs are compiled into byte codes for a
  (hardware-independent) Java Virtual Machine, and
  is executed by an interpreter.
• Just-In-Time compiler converts byte codes into
  native codes, for efficiency.
• An interpreter for JVM verifies and then executes
  Java byte codes. This step incorporates Security
  Checks.

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Evolution of Suns JDK

• Java 1.0 Interpreter
• Java 1.1 Interpreter JIT Compiler
• Java 2 Hotspot
• Profiling and Adaptive Dynamic Compilation of
  hot code
• Method in-lining and other aggressive
  optimizations, and Decompilation
• Improved Memory Management for long-running
  (server) programs
• Fast Thread Synchronization

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Concurrent Programming

• Threads can share address space. In Java, threads
  are modeled after Hoares monitors.
• Java is a multi-threaded system, which is very
  convenient for coding interactive or multi-media
  applications.
• Doing I/O concurrently with reading a document.
  (downloading via browser)
• Periodic updating of the screen with data
  acquired in real-time. (sports/stocks ticker)

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Distributed Processing

• HTML and WWW contributed immensely to the sharing
  of (distributed) static data.
• CGI scripts enabled customization of responses
  based on user input (client-server).
• A Java applet can further react to user input and
  change behavior dynamically. It can provide an
  alternative to client-server model in that it
  runs on the client machine.

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(cont)

• Java supports network programming.
• Java supports mobile computing.
• Compiled to compact, platform independent byte
  codes
• Dynamic linking.
• KVM